The invention relates more particularly to a so-called "articulated" rotorcraft rotor as described in FR 2 456 034, FR 2 516 891 and FR 2 671 049.
The above patents disclose a rotorcraft rotor, such as a helicopter main rotor or tail rotor, which comprises:
a tubular pylon adapted to be driven in rotation about its axis, which is the rotation axis of the rotor, from its foot, PA1 a hub attached to the pylon by a flared base and comprising two annular plates substantially radial to the rotor axis and spaced from each other along the rotor axis so that relative to the pylon one is an axially inside plate and the other is an axially outside plate, PA1 at least two blades each of which has its root connected to the hub by retaining and articulation means retained between said plates of the hub and enabling relative angular movement of the blade at least in pitch about a longitudinal blade pitch change axis and controlled by displacement by a pitch rod of a pitch lever attached to the blade root.
Compared to articulated rotors with a single plate hub, as described in FR 2 427 251, or with an integrated tubular pylon-hub, as described in FR 2 584 996, dual plate hub articulated rotors of the type referred to above have the advantage of improved resistance to centrifugal forces and to shear forces and static and dynamic flapping and drag bending moments for substantially the same overall size, mass and aerodynamic drag.
Like the single plate hub or integrated tubular pylon-hub rotors described in FR 2 427 251 and FR 2 584 996, respectively, the dual plate hub articulated rotors described in FR 2 456 034, FR 2 516 891 and FR 2 671 049 can be equipped with pitch control devices entirely external to the pylon-hub assembly, generally in the case of main rotors, or entirely internal to the tubular pylon and to the hub connection base, generally in the case of tail rotors, or partly internal to the tubular pylon and external to the hub, with a collective pitch control plate usually called the spider plate axially external of the hub, in the particular case of tail rotors.
In pitch control devices entirely external to the pylon-hub assembly the pitch lever of each blade is articulated to one end of a pitch rod external to the base connecting the tubular pylon to the hub and the other end of which is articulated to a pitch control plate that surrounds the rotor pylon, with which this plate is constrained to rotate and around which this plate is mobile at least in the axial direction.
In the case of a main rotor this pitch control plate is generally the rotating plate of a set of cyclic plates in which the rotating plate is rotatably mounted on a non-rotating plate that can be moved in axial translation and inclined in any direction about the axis of the rotor by controls operated by the pilot.
The above pitch control devices are too complex, with too large an overall size around the rotor pylon and too high a mass to be economically transposed to controlling the collective pitch of the blades of a tail rotor, for which all that is required is to move the rotating pitch control plate axially.
For this reason pitch control devices for tail rotors internal to the pylon-hub assembly have been proposed that include a pitch control plate that rotates with the rotor and slides axially inside the base connecting the pylon to the hub, being articulated, for each blade, to a pitch rod that is also internal of the assembly and is itself articulated to a pitch lever on a radial inner armature of the means for retaining each blade on the hub and articulating it thereto, so that the pitch lever is also inside the pylon-hub assembly.
This complicates the structure of the radially inner armature and increases its overall size and its mass, and therefore its cost, as well as increasing the eccentricity of flapping of the rotor and its aerodynamic drag.
What is more, placing the pitch rods inside the pylon-hub assembly complicates maintenance and makes it more costly, in that checking the pitch rod ball joints for play is complicated, as is any work to be carried out on the ball joints in order to replace them.
What is more, the dimensions and therefore the mass and the drag of a tail rotor of the above kind with internal pitch control are further increased if the dimensions of the spherical laminated thrust bearing included in the means for retaining each blade on and articulating it to the hub of an articulated rotor of the above kind must be increased to increase the service life of those units, which accentuates the drawbacks associated with the use of internal pitch controls.
The problem to which the invention is directed is that of remedying the aforementioned disadvantages and proposing an articulated rotor of the type described hereinabove equipped with a pitch control device that is better suited to the various requirements of the art than the prior art devices, in particular for tail rotors.